Rechargeable metal-ion batteries, for example lithium ion batteries, are extensively used in portable electronic devices such as mobile telephones and laptops, and are finding increasing application in electric or hybrid electric vehicles.
With reference to FIG. 1, known rechargeable metal ion batteries 100 have a conductive layer 101 such as a layer of metal; an anode layer 103; a cathode layer 107 capable of releasing and re-inserting metal ions; an electrolyte 105 between the anode and cathode layers 103 and 107; and a conductive layer 109 such as a layer of metal. When the battery cell is fully charged, metal ions have been transported from the metal-ion-containing cathode 107 layer via the electrolyte 105 into the anode layer 103.
The anode layer 103 may contain particles of an electroactive material and a binder material (“active material” or “electroactive material” as used anywhere herein means a material which is able to insert into its structure, and release therefrom, metal ions such as lithium, sodium, potassium, calcium or magnesium during the respective charging phase and discharging phase of a battery. Preferably the material is able to insert and release lithium.)
In the case of a graphite-based anode layer of a lithium ion battery, lithium insertion results in formation of the compound LixC6 (0<=x<=1). Graphite has a maximum capacity of 372 mAh/g.
The use of a silicon-based active anode material is also known in the art. Silicon has a substantially higher maximum capacity than graphite. However, unlike active graphite which remains substantially unchanged during insertion and release of metal ions, the process of insertion of metal ions into silicon results in substantial structural changes, accompanied by substantial expansion. For example, insertion of lithium ions into silicon results in formation of a Si—Li alloy. The effect of Li ion insertion on the anode material is described in, for example, “Insertion Electrode Materials for Rechargeable Lithium Batteries”, Winter et al, Adv. Mater. 1988, 10, No. 10, pages 725-763.
US 2009/301866 discloses a multilayer of a solid support, a first solid layer adhering to the solid support and a second solid layer adhering to the first solid layer, wherein each of the first and second solid layers contain particles of an electrochemically active material and a binder. Both the first and second layer comprise an elastomeric binder.
US 2012/040242 discloses an anode of a lithium ion secondary battery, the anode having a multilayer structure composed of a first layer containing silicon and a second layer containing silicon and a metal element. The presence of the metal element is stated to inhibit expansion and shrinkage of the anode.
U.S. Pat. No. 7,311,999 discloses an anode of an anode collector, an anode active material layer and a layer of silicon oxide on the anode active material layer. The layer of silicon oxide is included to inhibit reaction between the anode active material layer and an electrolyte.
U.S. Pat. No. 7,638,239 discloses an electrode of a current collector containing copper, an active material and a buffer formed from two layers between the current collector and the active material. The buffer is provided to prevent excessive diffusion of copper from the current collector into the active material, and diffusion of silicon from the active material into the current collector.
U.S. Pat. No. 7,824,801 discloses an electrode of a current collector, a first silicon layer having no oxygen or a low ratio of oxygen to silicon and a second silicon layer having a higher oxygen to silicon ratio. The first layer is stated to have a high charge/discharge capacity and high electron conductivity, but a large expansion coefficient and low ion conductivity. The second layer is stated to have a smaller coefficient of expansion and lower charge/discharge capacity than the first layer, but higher ion conductivity.
U.S. Pat. No. 8,080,337 discloses a current collector and alternating first and second layers. The first layer contains an active material. The second layer has a larger Young's modulus than the first layer and is conductive. The second layer may be a conductive metal compound, for example a metal nitride, a metal carbide or a metal boride.
Preferably the invention provides energy generating devices, including but not limited to metal ion batteries, having improved performance.
General
According to a first aspect of the invention, there is provided a multilayer electrode comprising, in sequence, a conductive layer, a first composite electrode layer and second composite electrode layer, each composite electrode layer comprising a particulate material suitable for use as the active material in a metal ion battery and a binder, wherein a major component of the active material of the first composite electrode layer (first major active component) is a material that is different from a material forming a major component of the active material of the second composite electrode layer (second major active component).
Optionally the multi-layer electrode comprises an interface between the first composite layer and the second composite layer.
Optionally, the binders of the first and second composite electrode layers are different.
Optionally, the binder of the first composite electrode layer is an elastomeric polymer and the binder of the second composite electrode layer is a non-elastomeric polymer.
Optionally, the theoretical specific capacity of the major active component of the second composite electrode layer is higher than that of the major active component of the first composite electrode layer.
Optionally one or each-layer comprises one or more sub-layers.
Optionally the first composite electrode layer comprises one or more lower sub-layers adjacent to or in the region of the current collector. Optionally the first composite electrode layer comprises one or more upper sub-layers at or in the region of the interface between the first composite electrode layer and the second composite electrode layer.
Optionally the second composite layer comprises a lower surface adjacent the interface and an upper surface distal the interface.
Optionally the second composite layer comprises one or more lower sub-layers adjacent to or in the region of the interface between the first composite electrode layer and the second composite electrode layer. Optionally the second composite layer comprises one or more upper sub-layers at or in the region of an upper surface of the second composite electrode layer.
Optionally the concentration of the major active component of one sub-layer differs from the concentration of a major active component in an adjacent sub-layer within a composite layer of the multi-layer electrode.
Optionally the concentration of the first major active component decreases between the current collector and the interface with the second composite electrode layer.
Optionally the concentration of the second active component increases in a direction between the interface with the first composite electrode layer and an upper surface of the second composite electrode layer.
Optionally the composition of the binder in one sub-layer differs from the composition of the binder in an adjacent sub-layer within a layer of the multi-layer electrode. Optionally the binder comprises a co-polymer of an elastomeric and a non-elastomeric polymer. Optionally the binder comprises a co-polymer comprising an elastomeric and a non-elastomeric polymer in a ratio 90:10 to 10:90.
Optionally the porosity of the first composite electrode layer is different to the porosity of the second composite electrode layer.
Optionally the porosity of the first composite electrode layer is less than the porosity of the second composite electrode layer.
Optionally the porosity of the first composite electrode layer is greater than 5 vol %.
Optionally the porosity of the first composite electrode layer is less than 30 vol %.
Optionally the porosity of the first composite electrode layer is in the range 20 to 25 vol %.
Optionally the porosity of the second composite electrode layer is greater than 20 Vol %.
Optionally the porosity of the second composite electrode layer is less than 80 Vol %.
Optionally the porosity of the second composite layer is in the range 30 to 70 vol %.
Optionally the porosity of one sub-layer differs from the porosity of an adjacent sub-layer within a layer of the multi-layer electrode.
Optionally the porosity of an upper sub-layer of the second composite electrode layer is less than the porosity of a lower sub-layer.
Optionally the first composite electrode layer comprises a first major active component comprising particles of the same or similar morphologies. Optionally the first composite electrode layer comprises a first major active component comprising particles of different morphologies.
Optionally the second composite electrode layer comprises a second major active component comprising particles of the same or similar morphologies.
Optionally the second composite electrode layer comprises a second major active component comprising particles of different morphologies.
Optionally the major active component of the first composite layer comprises an electroactive carbon. Optionally the electroactive carbon is natural and/or artificial graphite or hard carbon.
Optionally, the major active component of the second composite electrode layer is selected from the group consisting of silicon, tin, aluminium, lead and antimony.
Optionally, the theoretical specific capacity of the major active component of the second composite electrode layer is greater than 500 mAh/g and the theoretical specific capacity of the major active component of the first composite electrode layer is less than 400 mAh/g.
Optionally the theoretical specific capacity of one sub-layer differs from the theoretical specific capacity of an adjacent layer within a layer of the multi-layer electrode.
Optionally the theoretical specific capacity of an upper sub-layer of a second composite electrode layer is greater than the theoretical specific capacity of a lower sub-layer.
Optionally, the concentration of the major active component in the first composite anode layer is higher than the concentration of the major active component in the second composite anode layer.
Optionally, the volume increase V1 of the major active component of the second composite electrode layer is at least 90%.
Optionally, the second composite electrode layer contains no more than 20 grams per square meter of the major active component of that layer.
Optionally, the volume increase V1 of the major active component of the first composite anode layer is no more than 30%.
Optionally, the second composite electrode layer contains at least 30 grams per square meter of the major active component of that layer.
Optionally, doped or undoped silicon is the major active component of the second composite electrode layer and active carbon is the major active component of the first composite electrode layer.
Optionally, the active carbon is selected from one or more of hard carbon, carbon nano-tubes and graphite.
Optionally the graphite comprises natural or synthetic graphite. Optionally the graphite is provided in the form of flakes, meso-carbon micro-beads and massive artificial graphite. Small, medium and large carbon flakes may optionally be utilized.
Optionally the graphite comprises meso-carbon micro-beads.
Optionally the active carbon comprises elongate such as carbon nano-tubes and carbon fibres.
Optionally the active carbon comprises hard carbon. Optionally, active carbon is the only major active component of the first composite electrode layer
Optionally, doped or undoped silicon is the only active component of the second composite electrode layer.
Optionally the active silicon comprises flakes, particles, fibres, ribbons, scaffold structures, tubes and a mixture thereof. Optionally, the active silicon particles comprise native particles, pillared particles, porous particles, porous particle fragments and mixtures thereof.
Optionally, the particles are spheroidal, cuboidal, elongate or spherical in shape. Optionally, the second composite electrode layer comprises at least one further active material.
Optionally the further active material is an active carbon material.
Optionally, the binder of the first composite electrode layer is PVDF. Optionally the binder for the first composite layer is a polyimide.
Optionally, the binder of the second composite electrode layer is PAA or a salt thereof.
Optionally the binder for second composite electrode layer is carboxymethylcellulose (CMC) or a salt thereof, styrene butadiene rubber (SBR) or a binary or tertiary mixture thereof.
Optionally the binder of the first composite layer comprises a mixture or a copolymer of PVDF and PAA, wherein the PVDF and the PAA are present in a range of 90:10 to 55:45.
Optionally the binder of the second composite layer comprises a mixture or a copolymer of PVDF and PAA, wherein the PVDF and PAA are present in a range of 10:90 to 45:55.
Optionally the binder of either the first composite layer or the second composite layer comprises a mixture of PVDF and PAA. Optionally the binder of the first composite layer comprises PVDF and PAA, wherein the PVDF and PAA are present in a ratio of 90:10 to 55:45. Optionally the binder of the second composite layer comprises PVDF and PAA, wherein the PVDF and PAA are present in a ratio of 10:90 to 55:45.
Optionally the binder of either the first composite layer or the second composite layer comprises a mixture of polyimide (PI) and carboxymethylcellulose (CMC) or styrene butadiene rubber (SBR). Optionally the binder of the first composite layer comprises Polyimide and CMC/SBR, wherein the PI and CMC/SBR are present in a ratio of 90:10 to 55:45. Optionally the binder of the second composite layer comprises PI and CMC/SBR, wherein the PI and CMC/SBR are present in a ratio of 10:90 to 55:45.
Optionally, the first composite electrode layer is formed on the conductive layer.
Optionally, the second composite electrode layer is formed on the first composite electrode layer.
Optionally, an adhesion layer is provided between the first and second electrodes.
Optionally the adhesion layer comprises a mixture of an elastomeric and a non-elastomeric polymer.
Optionally an adhesion layer further comprises a conductive carbon. Optionally the conductive carbon comprises carbon fibres, carbon nano-tubes, ketjen black, lamp black, acetylene black, pitch black, graphene and mixtures thereof.
Optionally, one or more of the composite electrode layers further comprises a conductive particulate additive. Optionally the conductive particulate comprises carbon fibres, carbon nano-tubes, ketjen black, lamp black, acetylene black, graphene and pitch black.
In a second aspect of the invention, there is provided a metal ion battery comprising an anode according to the first aspect of the invention.
In a third aspect of the invention there is provided a method of forming a multilayer electrode according to the first aspect of the invention comprising the steps of: forming the first composite electrode layer over the conductive layer; and forming the second composite electrode layer over the first composite electrode layer.
Optionally, the first and second composite electrode layers of the third aspect may each be formed by depositing a slurry comprising the components of the composite electrode layer and one or more solvents, and evaporating the one or more solvents.
Optionally an adhesion layer is deposited between the first composite layer and the second composite layer. Optionally the adhesion layer comprises carbon fibres, polymeric fibres, metal fibres or a mixture thereof.
Optionally the first and second composite electrode layers are deposited using one or more methods selected from doctor blade coating, electrostatic coating techniques including powder coating, spin coating, spray coating, vertical coating, dip coating and chemical vapour deposition.
Optionally, in the third aspect of the invention, pressure may be applied to at least the first composite electrode layer during or after evaporation of the one or more solvents.
Optionally, in the third aspect of the invention, pressure may be applied by calendering.
Optionally, in the third aspect of the invention, a first pressure may be applied to the first composite electrode layer prior to formation of the second composite electrode layer, and no pressure or a second pressure may be applied to the second composite electrode layer wherein the second pressure is lower than the first pressure.
Optionally, in the third aspect of the invention, the binder of the first composite electrode layer is insoluble in the solvent or solvent mixture of the slurry used to form the second composite electrode layer.